The only draw back of the amped kits is they do not warranty if you use a 48v battery. If you want more speed you have to go to a higher battery voltage. I would go with the ebikekit which is set up for 48v out of the box.

http://www.e-bikekit.com/shop/index....d=110&parent=4

The geared motors have a free wheel, so you can pedal without any drag. There is a little drag in the direct drive motors.

To fit the larger cassettes you might have to spread the drop outs a little. That can be a problem if you have aluminum drop outs.